In the manufacture of composite elements it is often desirable, to obtain the desired structural properties, that a certain thickness, is applied to the element, however, preferably without considerably increasing the weight of the element. This can be obtained by sandwich constructions wherein a comparatively thick and light layer of core material is delimited to both sides by one or more more rigid and stronger layers. As core material, polymer foam is often used, such as foamed PVC, balsa wood or a material having a honeycomb structure. However, those materials are associated with a number of drawbacks. On the one hand, they are comparatively expensive materials and, on the other, it is difficult to adapt the materials to composite elements of rather complex geometries. In particular curved or—even worse—double-curved surfaces present a problem. Here the core material can either be cut to the right dimensions and shapes which, on the one hand, is a very time-consuming and work-intensive process and, on the other, it involves a considerable waste of material, or one may make sections from the one or both sides of the core material, whereby it is more readily adapted to a curved surface. However, the latter entails that the areas where the sections are opened and form wedges are filled with resin during moulding of the composite element which will, in its finished state, exhibit varying strength and rigidity properties through the material.
WO 2004/087410 teaches a fibre-reinforced structure for manufacturing composite elements consisting of two fibre-reinforced layers on each side of a core layer. Here the core layer is made of a quantity of elongate fibre on or between which separate bodies, such as eg hollow glass beads, are secured. The core layer is combined with the fibre layers by sewing. This core layer is more flexible than the above core materials, such as eg balsa wood, but still the core layer cannot be adapted to curved or double-curved surfaces without having to be cut into suitable pieces.
EP 0596191 A1 discloses a fibrous laminated material used as cover layer above fluid absorbing materials, e.g. in connection with diapers, sanitary towels, patches, etc. The fibrous laminate comprises two layers of fibre material put together. The two layers are bonded together in a pattern by passing the layers through two bonding rolls. The two fibre layers are between the bonding areas only put together and thus they are forming a cavity. The bonding areas comprise openings which allow fluid to flow through the fibrous laminate material, and fluid can therefore only flow through the openings in the bonding areas and not into the cavity. Nothing is mentioned about core material in EP 0596191 A1 and the disclosed fibrous laminated material cannot be used as core material.
EP1010793 A discloses a core material to be used in connection with production of fibre reinforced plastic materials in closed moulds. The core material comprises a fibre web comprising a closed cell foam structure comprising 40 vol. % or more and has a free volume accessible by resin of less than 60% vol. The core material can be prepared by printing a foamed or unfoamed binder with micro-spheres (size 4-20 μm) in a non-woven fibre layer. The fibre layer is hereafter dried and heated with the result that the micro-spheres expand (size: 10-100 μm), the binder material cures and sets the micro-spheres in the web, and the micro-spheres would thereafter constitute the closed cell foam structure of the core material. The micro-spheres can further be introduced into the fibre web by using impregnation techniques. Preferably the micro-spheres are placed in regular patters in the fibre web as to form “islands” which are separated by areas (channels) containing no micro-spheres and hereby resin flow into the core material is allowed. The micro-spheres are set in the fibre web and the consequence is that the core material therefore needs to be formed so that it fits the construction prior to use.